1. Field of the Invention
The present invention relates to a mobile communication technology and an uplink transmission control method, and more particularly, to a method for allocating a Physical Hybrid Automatic Repeat Request Indicator Channel (PHICH) and generating a reference signal in a system using Single-User Multiple Input Multiple Output (SU-MIMO) based on multiple codewords upon uplink transmission.
2. Discussion of the Related Art
In a mobile communication system, a user equipment (UE) may receive information from a base station (BS) in downlink and transmit information in uplink. The information transmitted or received by the UE includes data and a variety of control information, and a physical channel varies according to the type of information transmitted or received by the UE.
FIG. 1 is a view showing physical channels used for a 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) system, which is an example of a mobile communication system, and a general signal transmission method using the same.
When a UE is powered on or when the UE newly enters a cell, the UE performs an initial cell search operation such as synchronization with a BS in step S101. In order to perform the initial cell search, the UE may receive a Primary Synchronization Channel (P-SCH) and a Secondary Synchronization Channel (S-SCH) from the BS so as to perform synchronization with the BS, and acquire information such as a cell ID. Thereafter, the UE may receive a physical broadcast channel from the BS and acquire broadcast information in the cell. Meanwhile, the UE may receive a Downlink Reference signal (DL RS) in the initial cell search step and confirm a downlink channel state.
The UE, upon completes the initial cell search, may receive a Physical Downlink Control Channel (PDCCH) and a Physical Downlink Shared Channel (PDSCH) corresponding to the PDCCH, and acquire more detailed system information in step S102.
Meanwhile, if the UE does not complete access to the BS, the UE may perform a random access procedure in steps S103 to S106, in order to complete access to the BS. In order to perform a random access procedure, the UE may transmit a feature sequence via a Physical Random Access Channel (PRACH) as a preamble (S103), and may receive a response message to the random access procedure via the PDCCH and the PDSCH corresponding thereto (S104). In contention-based random access, except for handover, a contention resolution procedure including transmission of an additional PRACH (S105) and reception of the PDCCH and the PDSCH corresponding thereto (S106) may be performed.
The UE, having performed the above-described procedure, may then receive the PDCCH/PDSCH (S107) and transmit a Physical Uplink Shared Channel (PUSCH)/Physical Uplink Control Channel (PUCCH) (S108), as a general uplink/downlink signal transmission procedure.
FIG. 2 is a view explaining a signal processing procedure for enabling a UE to transmit an uplink signal.
In order to transmit the uplink signal, a scrambling module 210 of the UE may scramble a transmitted signal using a UE-specific scrambling signal. The scrambled signal is input to a modulation mapper 220 so as to be modulated into complex symbols using Binary Phase Shift Keying (BPSK), Quadrature Phase Shift Keying (QPSK) or 16-Quadrature amplitude modulation (QAM) according to the kind of the transmitted signal and/or the channel state. Thereafter, the modulated complex symbols are processed by a transform precoder 230, and the processed complex symbols are input to a resource element mapper 240. The resource element mapper 240 may map the complex symbols to time-frequency resource elements used for actual transmission. The signal processed as described above may be transmitted to a BS via an SC-FDMA signal generator 250 and an antenna.
FIG. 3 is a view explaining a signal processing procedure for enabling a BS to transmit a downlink signal.
In the 3GPP LTE system, the BS may transmit one or more codewords in downlink. Accordingly, one or more codewords may be processed by scrambling modules 301 and modulation mappers 302 to configure complex symbols, similar to the uplink transmission of FIG. 2. Thereafter, the complex symbols are mapped to a plurality of layers by a layer mapper 303, and each layer may be multiplied by a predetermined precoding matrix, which is selected according to the channel state, by a precoding module 304 and may be allocated to each transmission antenna. The processed signals which will respectively be transmitted via antennas may be mapped to time-frequency resource elements used for transmission by resource element mappers 305, and may respectively be transmitted via OFDM signal generators 306 and antennas.
In a mobile communication system, in a case where a UE transmits a signal in uplink, a Peak-to-Average Ratio may be more problematic than the case where a BS transmits a signal in downlink. Accordingly, as described above with reference to FIGS. 2 and 3, downlink signal transmission uses an OFDMA scheme, while uplink signal transmission uses an SC-FDMA scheme.
FIG. 4 is a diagram explaining an SC-FDMA scheme for uplink signal transmission and an OFDMA scheme for downlink signal transmission in a mobile communication system.
A UE for uplink signal transmission and a BS for downlink signal transmission are identical in that a serial-to-parallel converter 401, a subcarrier mapper 403, an M-point Inverse Discrete Fourier Transform (IDFT) (or IFFT) module 404 and a Cyclic Prefix (CP) adding module 406 are included.
The UE for transmitting a signal using an SC-FDMA scheme further includes a parallel-to-serial converter 405 and an N-point DFT module 402. The N-point DFT module 402 performs mapping to contiguous input points in an input unit of IDFT and partially offsets an IDFT (or IFFT) process influence of the M-point IDFT (or IFFT) module 404 such that the transmitted signal has a single carrier property.
A channel for transmitting ACKnowledgement (ACK)/Negative ACKnowledgement (NACK) for uplink data transmission (Physical Uplink Shared CHannel (PUSCH)) in downlink is referred to as a Physical Hybrid Automatic Repeat Request Indicator CHannel (PHICH) in a 3GPP LTE system. FIG. 5 is a diagram illustrating a process of transmitting a PHICH in a 3GPP LTE system.
Since an LTE system does not use SU-MIMO in uplink, only 1-bit ACK/NACK for PUSCH transmission of one UE, that is, a single data stream or codeword, is transmitted through a PHICH. The 1-bit ACK/NACK is encoded into 3 bits using repetition coding with a code rate of ⅓ (step 501), and three modulation symbols are generated using Binary Phase Shift Keying (BPSK) (step 502). The modulation symbols are spread using a Spreading Factor (SF) of 4 in the case of normal cyclic prefix and are spread using an SF of 2 in the case of extended cyclic prefix (step 503). The number of orthogonal sequences used for spreading becomes SF*2 in terms of In-phase/Quadrature (I/Q) multiplexing concept. Accordingly, SF*2 PHICHs spread using SF*2 orthogonal sequences are defined as one PHICH group and PHICH groups located in a certain subframe are layer-mapped (step 504), precoded, resource-mapped (step 505), and then transmitted.
In a method for allocating downlink PHICH channel resources of a cell, or a BS or a relay node to uplink data transmission of certain user equipments or relay nodes, using a computation process using a lowest Physical Resource Block (PRB) index of one or more PRBs used for transmission of a PUSCH and a cyclic shift value set as resources for a Demodulation Reference Signal (DM-RS) used for the channel transmission, a PHICH group index used for transmission out of all PHICH groups and a PHICH channel index within the PHICH group are derived, and PHICH channels which will be transmitted to the certain UEs or relay nodes are allocated using these indexes. A MIMO scheme may remarkably increase system capacity by simultaneously and spatially transmitting several data streams (or codewords) using two or more transmission and reception antennas at a BS and a terminal and may obtain transmit diversity gain or beamforming gain using several transmission antennas. In a transmit diversity scheme, since the same data information is transmitted through several transmission antennas, it is possible to perform data transmission with high reliability in a channel state which is rapidly changed with time and to perform data transmission without feedback information associated with a channel. Beamforming is used to increase a Signal to Interference plus Noise Ratio (SINR) of a receiver by applying respective adequate weights to several transmission antennas. In general, in a Frequency Division Duplexing (FDD) system, since uplink and downlink channels are independent, high reliability channel information is necessary to obtain appropriate beamforming gain. Accordingly, separate feedback is received and used from the receiver.
FIG. 6 is a diagram illustrating Spatial Multiplexing (SM) and Spatial Division Multiple Access (SDMA). SM for a single user is referred to as SM or SU-MIMO. Channel capacity of a MIMO system increases in proportion to a minimum value among the numbers of transmission/reception antennas. SM for multiple users is referred to as Spatial Division Multiple Access (SDMA) or Multi-User MIMO (MU-MIMO).
When using the MIMO scheme, there are a Single CodeWord (SCW) scheme for simultaneously transmitting N data streams using one channel encoding block and a Multiple CodeWord (MCW) scheme for transmitting N data streams using M (M is always equal to or less than N) channel encoding blocks. Each channel encoding block generates an independent codeword and each codeword is designed for independent error detection.
FIG. 7 is a diagram showing the structure of a transmitter of a MIMO system using a MCW scheme. In detail, M data packets are subjected to encoding (e.g., turbo encoding of FIG. 7) and modulation (e.g., QAM modulation of FIG. 7) so as to generate M codewords, and each codeword has an independent HARQ process block. The M modulated data symbols are simultaneously encoded in a MIMO stage according to a multi-antenna scheme and are transmitted through respective physical antennas. Thereafter, a receiver feeds back a multi-antenna channel state as channel quality information so as to control an SM rate, a coding rate and a modulation scheme. In this case, additional control information is necessary.
A mapping relationship between codewords and physical antennas has a certain format.
FIG. 8 is a diagram showing an example of a mapping relationship between codewords and physical antennas. Specifically, FIG. 8 shows codeword-to-layer mapping for SM rate in downlink (DL) of 3GPP TS 36.211. As shown in FIG. 8, if SM rate (that is, rank) is 1, one codeword is mapped to one layer, data of one layer is encoded using a precoding scheme so as to be transmitted through four transmission antennas. If SM rate is 2, two codewords are mapped to two layers and are mapped to four antennas by a precoder.
If SM rate is 3, one of two codewords is mapped to two layers by a serial-parallel (S/P) converter, two codewords are mapped to three layers and is mapped to four antennas by a precoder. If an SM rate is 4, two codewords are mapped to two layers by an S/P converter and a total of four layers is mapped to four antennas by a precoder. That is, in the case of a BS having four transmission antennas, a maximum of four layers may be used and four independent codewords may be used. However, in FIG. 8, the number of codewords is a maximum of two. Accordingly, in the system shown in FIG. 8, if each codeword CW has an independent HARQ process, a maximum of two independent HARQ processes may be used.
Currently, in the LTE system, on the assumption that a single RF and a power amplifier chain are used in PUSCH transmission, since channel assignment of a downlink PHICH to a PUSCH is designed based on 1-bit ACK/NACK per UE, there is a need for improvement in channel capacity and assignment method in consideration of SU-MIMO based on multiple codewords in PUSCH transmission.